A spark plug is mounted to an internal combustion engine (sometimes just referred to as “engine”) and used for ignition of an air-fuel mixture in a combustion chamber of the engine. In general, the spark plug includes an insulator formed with an axial hole, a center electrode inserted in a front side of the axial hole, a metal shell arranged circumferentially around the insulator and a ground electrode joined to a front end portion of the metal shell. The ground electrode has a bent portion formed at a substantially middle position thereof in such a manner that a distal end portion of the ground electrode faces a front end portion of the center electrode so as to define a spark discharge gap between the distal end portion of the ground electrode and the front end portion of the center electrode. With the application of a high voltage to the center electrode, a spark discharge occurs in the spark discharge gap to ignite the air-fuel mixture (see, for example, Japanese Laid-Open Patent Publication No. 2008-108478). For improvement in corrosion resistance, the metal shell joined with the ground electrode may be coated with a Ni plating layer or zinc plating layer by a barrel plating machine etc.
There has recently been a demand to reduce the thickness of the ground electrode so that the ground electrode can be joined to the diameter-reduced metal shell for the purpose of size and diameter reduction of the spark plug. However, such a thin ground electrode may be bent or twisted during the process of applying the Ni plating layer to the ground electrode or the process of joining the ground electrode to the metal shell. Further, it is unlikely in the relatively thin ground electrode that heat will be transferred from the distal end portion of the ground electrode to the metal shell. This can result in quick wearing of the distal end portion of the ground electrode during use.
The present invention has been made in view of the above circumstances. An advantage of the present invention is a spark plug having a relatively thin ground electrode configured to obtain improvements in both of deformation resistance and wear resistance. Another advantage of the present invention is a method for manufacturing such a spark plug.